Battery woes

[Martin Butler]

Hi, , everyone, somethings gone wrong on my car, went out Wednesday to find my battery flat, so i have a fully charged spare, fitted it Friday morning, did one run, to the supermarket, and, i find this morning , its flat again, no lights ,or anything left on, so there should have been minimal drain, the first battery is only a month old, the spare is the one i took off about 3 years old, i have checked all earths and their all clean and tight, any ideas what could drain the battery so fast, Also when i fitted the other battery, normally the engine fires straight up, but this time, it spun over a couple of times before it fired,